2. Control UnitControl Unit
The control information is stored in a control memory, and the control
memory is programmed to initiate the required sequence of
microoperations
+Any required change can be done by updating the microprogram
incontrol memory.
-Slow operation
The control logic is implemented with gates, F/Fs, decoders, andother
digital circuits
+ Fast operation.
-Wiring change(if the design has to be modified)
»Microprogrammed Control : in this Chapter
»Hardwired Control : in Chap. 5
groups of bits that select the paths in multiplexers, decoders, and
arithmetic logic units
Control signal (that specify microoperations)in a bus-organized system
• Two major types of Control Unit
• Initiate sequences of microoperations
3. Control WordControl Word
The control variables at any given time can be represented by a string
of 1’s and 0’s.
A control unit whose binary control variables are stored in memory
(control memory).
Microprogrammed Control UnitMicroprogrammed Control Unit
MicroinstructionMicroinstruction
MicroprogramMicroprogram
The microinstruction specifies one or more microoperations
A sequence of microinstruction
RAM can be used for writing (to change a writable control
memory)
Microprogram is loaded initially from an auxiliary memory such
as a magnetic disk
Control words in ROM are made permanent during the
hardware production.
»Dynamic microprogramming :Control Memory =RAM
»Static microprogramming : Control Memory =ROM
4. Microprogrammed Control OrganizationMicroprogrammed Control Organization
Next-address
generator
(sequencer)
Control
address
register
Control
memory
(ROM)
Control
data
register
Control
word
Next-address information
External
input
Control Memory
»A memory is part of a control unit :
»Computer Memory (employs a microprogrammed control unit)
•Main Memory : for storing user program (Machine instruction/data)
•Control Memory : for storing microprogram (Microinstruction)
5. Control Address Register
Sequencer
Control data register
»Specify the address of the microinstruction
»Determine the address sequence that is read from control
memory
»Next address of the next microinstruction can be specified several
way depending on the sequencer input.
»Hold the microinstruction read from control memory
»Allows the execution of the microoperations specified by the
control word simultaneously with the generation of the next
microinstruction
1. Incrementing of the control address register
2. Unconditional and conditional branches
3. A mapping process from the bits of the machine instruction
to an address for control memory
4. A facility for subroutine call and return
Sequencing Capabilities Required in a Control Storage
